According to the US Dept. of Energy, the average PC + Monitor uses ((120+150 Watts) x 4 hours/day x 365 days/year)/1000 = 394 kWh
So, you have the average PC using 270 watts. 1 watt is 3.412 btu/h. 270*3.412 = 921.24 btu/h
This depends on the hardware and is different for every computer. For specificity, you need to get the actual watt usage for the PC models you’re using.
LCD monitors use considerably less than CRT monitors referred to in the above example.
Here is a table comparing common monitors:
From that you can expect to use 20 watts for anyLCD less than 20″ and no more than 35 watts even for 24″ LCD’s. Of course a dual monitor setup would also be double the energy.
For modern desktops, 2008 era Dell and Apple specifically, see this article a PennU:
This lists workstations at roughly 80 to 90 watts without LCD.
For better energy efficiency, consider using laptops with LCD screens.
The same PennU article shows ranging from 20 to 50 watts.
The USDE listing above appears to be for residential. Most business PC’s are used at least 8 hours per day. A call center may well be staffed 24/7.
So using 80 + 30 watts for a modern PC and medium sized LCD
= 110 watts x 8hrs/day x 365/1000 = 321 kWh annually
= 110 watts x 3.412 btu/h = 375 btu/hr